How to overcome the demagnetization of superconducting Halbach arrays?

Abstract

Assembling trapped-field superconducting magnets with mutually orthogonal magnetizations directions in a Halbach array configuration offers the prospect of generating both high fields and large field gradients. A major issue when assembling bulk superconductors in a Halbach array, however, consists in the alteration of the initial current density distribution during the assembly process. This reorganization of supercurrent loops limits the field generated by the system. We investigate two methods for reducing this demagnetization effect. The first method consists of using stacked tapes instead of bulk superconductors. For the second method, we propose a procedure leading to a re-magnetizing the superconductors of the array after the assembly. The procedure consists in putting two superconductors on top of one another, magnetizing them along the vertical direction, and then keeping the pair in place while two other superconductors, magnetized in an horizontal direction, are approached from left and right. The top central sample is then removed from the array, thereby providing the desired re-magnetization of the bottom one. The benefits of this procedure was investigated by finite element modelling and experiments carried out at 77 K both with bulk YBa2Cu3O 7 − x superconductors ( ∼ 14 × 14 × 14 mm3) and with stacks of second generation YBa2Cu3O 7 − x tapes from Superpower ( ∼ 12 × 12 × 12 mm3). The flux density measured above the array is compared to analytical results and finite element simulations. The results show that a re-magnetization of the central sample occurs, which allows the maximum field generated with Halbach arrays made of three bulk superconductors or three stacked tapes to be increased by 5% and 11% respectively. Numerical modelling shows that using a taller top sample with this method allows to recover almost the full potential of the array.

Enhancing the magnetic field gradient between two superconductors with rotational motion under a background DC field

“…In the previous section, the demagnetization of a superconductor rotated in a constant background field was investigated. In experiments involving two bulk superconductors, another potential source of current density alteration during rotational motion appears: the rather short distance between the magnetized superconductors during the rotation may result in a mutual demagnetization effect, similar to observations performed in the context of assembling superconducting Halbach arrays [37,58]. This effect should be overcome to achieve gradients when combining two superconductors in a background DC field.…”

“…The coupling between these formulations is achieved thanks to surface terms computed on the common boundary. This numerical tool was successfully adapted in our previous work [37] to model the translational motion of superconductors and it is extended in the present work to explore the rotation of superconducting samples. The rotation is modelled through the iterative adjustment of the position of superconducting regions at discrete time intervals.…”